1. Field of the Invention
The present invention relates to a thermal transfer sheet and, particularly, to a thermal transfer sheet which has high sensitivity, is adaptable to high-speed printing and causes no problem in various performances such as storage stability, background soiling or the like. The present invention also relates to a thermal transfer method and a thermal transfer system using such a thermal transfer sheet.
2. Description of the Related Art
A melting type thermal transfer system has been known, in which an applicable thermal transfer sheet is composed of a substrate sheet and a coloring layer which is obtained by dispersing a coloring agent such as a pigment or a dye in a heat-meltable binder such as wax or a resin and supported by the substrate sheet such as a plastic film, and energy corresponding to image information is applied to the thermal transfer sheet by means of a heating device such as a thermal head, and thus the coloring agent is transferred from the thermal transfer sheet to an image-receiving sheet such as paper or a plastic sheet together with the binder. A print image formed by this melting type thermal transfer system has high density and superior sharpness and is therefore suitable for the recording of binary images such as characters and line drawings. In addition, when plural coloring layers such as yellow, magenta, cyan and black are printed and recorded on the image-receiving sheet in a superposed manner with the use of one or more thermal transfer sheets, a multicolor or full color image can be formed by subtractive color mixing.
When images such as a bar code for which strict standards as to line width, intervals between lines and length are established are printed by melting transfer system, it is required for a thermal transfer sheet to be provided with a coloring layer having good film-cuttability and to have sharp printing ability. Also, it is required for a print product to have abrasive resistance in order to enable exact reading by a bar code reader and prevent the print portion from being soiled by rubbing in the handling of the print product. For this, many types of thermal transfer sheet provided with a coloring layer primarily comprising a resin such as an acrylic resin or vinyl acetate type resin have been proposed.
When information such as a bar code is recorded by thermal transfer in a transfer-receiving material by using a thermal transfer sheet of the melting type transfer system, high-speed processing (high-speed printing) is required in many applications. In this case, particularly a thermal transfer printer mounted with a so-called end face type thermal head, specifically, provided with a heat generating resistance part at an end surface of a substrate perpendicular to the plane on which a driver IC for driving is disposed is frequently used.
As the thermal transfer sheet used to attain high-speed processing (high-speed printing) in the above thermal transfer recording, one utilizing a material having a high sensitivity is used. However, it is difficult for a thermal transfer print product prepared by using such a thermal transfer sheet provided with a coloring layer made of a highly sensitive resin to be endowed with sufficient performances required for the print product such as durability, heat resistance or the like.
In general, an attempt to improve the coloring layer primarily composed of conventional resins in adaptability to high-speed transfer printing requires use of higher sensitive materials, namely materials having low softening points. However, the use of the material having a low softening point easily causes the so-called blocking phenomenon in which the coloring layer side of the thermal transfer sheet adheres to backface side of the thermal transfer sheet coming in contact with the coloring layer when winding the thermal transfer sheet in a coiled state (namely, a rolled form), or the background soiling via rubbing of the thermal transfer sheet against an image-receiving sheet during the thermal transfer printing, thus being inconvenient.
Japanese Patent Application Laid-open (Kokai) Nos. 5(1993)-104864 and 6(1994)-64339 disclose that a thermo-adhesive resin which is obtained by copolymerizing an xcex1-olefin, one or more kinds of monomer having an xcex1, xcex2-unsaturated double bond and maleic acid anhydride or by subjecting the resin obtained by such a copolymerizing process to esterification with alcohol is used for the adhesive layer or the coloring layer of the thermal transfer sheet to improve transferability to the image-receiving sheet and produce a print product excellent in adhesion property, wear resistance and outdoor durability. However, simple use of such a resin causes decrease of printing density and failure of sensitivity when carrying out the high-speed printing by means of a thermal transfer printer provided with an end face type thermal head.
On the other hand, attempts have been made to dispose a peelable layer or the like in combination with the coloring layer on a substrate of a thermal transfer sheet, in order to allow a print portion to have a multilayer structure and to enable prevention of the blocking phenomenon or the background soiling. For example, in these attempts, an adhesive layer having anti-blocking property or anti-background soiling property is disposed on the coloring layer. However, since one processing step in which the adhesive layer is formed by coating is added to the process for producing the thermal transfer sheet, the process become to require more cost and time, thus being troublesome.
Accordingly, it is an object of the present invention to solve the aforementioned problems by providing a thermal transfer sheet which is capable of being produced at a low cost, and excellent in adaptability to the high-speed printing, and cause no problem in storage stability in a coiled state, background soiling of the print product or the like.
In order to attain the above object, a thermal transfer sheet according to the present invention comprises a substrate and a coloring layer disposed on one side of the substrate, wherein the coloring layer is formed of a mixture comprising a copolymerization product (A) which is obtainable by polymerizing an xcex1-olefin/a maleic acid anhydride copolymer with a maleic acid anhydride monoester and an ethylene/vinyl acetate copolymer (B) which comprises a high viscous ethylene/vinyl acetate copolymer having a melt index in a range of 5 to 150 and a low viscous ethylene/vinyl acetate copolymer having a melt index in a range of 150 to 2400.
The melt index referred in the present invention is defined as the melt flow rate (MFR, unit:g/10 g) which is measured at a temperature of 190xc2x0 C. and a load of 2160 g under a condition in accordance with JIS K 7210. All measured values of MI (MFR) appearing hereunder will be described on the basis of this method.
In the present invention, since the above polymerization product (A) is contained in the coloring layer, a blocking phenomenon of the thermal transfer sheet during storage in a coiled state can particularly be prevented.
Further since the above copolymer (B) is contained in the coloring layer, decrease of the print density is not caused even at the high-speed printing, and a combination use of the high viscous and the low viscous ethylene/vinyl acetate copolymers further improves transferability of the coloring layer at the high-speed printing. Thus, the thermal transfer sheet is superior in the printing quality.
Further, since the copolymerization product (A) and the copolymer (B) are blended to form the coloring layer, a thermal transfer sheet which is superior in the storage stability in a coiled state, the anti-background soiling property and the adaptability to high-speed printing, and also capable of forming a print products having excellent functions as to durability such as wear resistance and heat resistance, and is well-balanced between various performances can be provided.
Further, it is preferable that a peelable layer containing wax as its primary component is further disposed between said coloring layer and said substrate.
The peelable layer is molten at the thermal transfer process and improves the peelability of the coloring layer from the substrate, and after transferred, at least a part of the peelable layer is transferred together with the coloring layer to cover the transferred coloring layer, it serves as a protective layer of the coloring layer and particularly imparts a good lubricity to a transferred image, thus improving anti-wear property.
The thermal transfer sheet of the present invention is preferably used in a printing system using a thermal transfer printer provided with an end face type thermal head. This makes it possible to carry out thermal transfer recording in high-speed printing.